fmt: add example for GoStringer interface

Updates golang/go#27376.

Change-Id: Ia8608561eb6a268aa7eae8c39c7098df100b643a
Reviewed-on: https://go-review.googlesource.com/133075Reviewed-by: Kevin Burke <kev@inburke.com>
Run-TryBot: Kevin Burke <kev@inburke.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/fmt/gostringer_example_test.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fmt_test
+
+import (
+	"fmt"
+)
+
+// Address has a City, State and a Country.
+type Address struct {
+	City    string
+	State   string
+	Country string
+}
+
+// Person has a Name, Age and Address.
+type Person struct {
+	Name string
+	Age  uint
+	Addr *Address
+}
+
+// GoString makes Person satisfy the GoStringer interface.
+// The return value is valid Go code that can be used to reproduce the Person struct.
+func (p Person) GoString() string {
+	if p.Addr != nil {
+		return fmt.Sprintf("Person{Name: %q, Age: %d, Addr: &Address{City: %q, State: %q, Country: %q}}", p.Name, int(p.Age), p.Addr.City, p.Addr.State, p.Addr.Country)
+	}
+	return fmt.Sprintf("Person{Name: %q, Age: %d}", p.Name, int(p.Age))
+}
+
+func ExampleGoStringer() {
+	p1 := Person{
+		Name: "Warren",
+		Age:  31,
+		Addr: &Address{
+			City:    "Denver",
+			State:   "CO",
+			Country: "U.S.A.",
+		},
+	}
+	// If GoString() wasn't implemented, the output of `fmt.Printf("%#v", p1)` would be similar to
+	// Person{Name:"Warren", Age:0x1f, Addr:(*main.Address)(0x10448240)}
+	fmt.Printf("%#v\n", p1)
+
+	p2 := Person{
+		Name: "Theia",
+		Age:  4,
+	}
+	// If GoString() wasn't implemented, the output of `fmt.Printf("%#v", p2)` would be similar to
+	// Person{Name:"Theia", Age:0x4, Addr:(*main.Address)(nil)}
+	fmt.Printf("%#v\n", p2)
+
+	// Output:
+	// Person{Name: "Warren", Age: 31, Addr: &Address{City: "Denver", State: "CO", Country: "U.S.A."}}
+	// Person{Name: "Theia", Age: 4}
+}